Multidimensional complex signals with single-orthant spectra

An extension of the notion of the analytical signal to multidimensional signals is presented. The real and imaginary parts of this signal are a linear combination of the original signal and of its complete and partial Hilbert transforms. Its Fourier image exists only in one orthant of the multidimensional frequency space. The orthant is a half-axis in one dimension, a quadrant in two dimensions, an octant in three dimensions, etc. A multidimensional complex signal makes it possible to introduce the definitions of instantaneous amplitude, instantaneous phase, and partial instantaneous complex frequencies (partial derivatives of the instantaneous phase) and to formulate a modulation theory of a multidimensional harmonic carrier. The 2-D equivalent of 1-D single-sideband modulation is defined and called single quadrant modulation. It is shown that the multidimensional complex signal with a signal orthant spectrum may be defined as a convolution of the real signal with the multidimensional complex delta distribution